Technical Field
The present invention relates to a method for producing fine powder of raw and processed materials that are used for the products in all sorts of technical fields, such as pharmaceutical products, cosmetics, paint, copiers, solar cells, secondary batteries and recording media. The present invention further relates to the fine powder produced by the present method. The present invention especially relates to a method for producing fine powder having significantly improved dissolvability and mixing uniformity.
Background Art
The existing candidate compounds for medicines often have a low solubility. The medicines of a low solubility is not absorbed effectively from digestive organs and is increased in dosage and also varied in absorption depending upon individual differences of patients, and thereby becomes difficult in making into a pharmaceutical product in some cases. In addition, particular medicines have a very small percentage of active ingredients in the medicines. In order to expect medical effects from medicines, therefore, it is important to secure the content uniformity of active ingredients in a pharmaceutical preparation.
The fine or impalpable powder has been produced by pulverizing various forms of raw materials such as particulate and powdery materials in smaller size and/or by dispersing aggregated particles in the raw material. As such a method for producing the fine or impalpable powder as mentioned above, a dry pulverization method represented by a jet mill and a hammer mill, and a wet medium pulverization method using a solid medium for pulverization such as a ball mill and a sand mill and a bead mill have been used. In the wet medium pulverization method using a bead mill, a slurry including the raw materials is agitated in a vessel together with a number of beads, each of which is formed by a sphere having a diameter of a few hundred microns to a few millimeters, and the raw materials are pulverized to become a fine or impalpable powder, for example, by a collision of a number of beads moving in the slurry and by a dispersion of secondary aggregated particles. As the beads for pulverization or dispersion, for example, ceramic beads made of hard and chemically stable zirconia, resin beads made of urethane or nylon that can reduce metal contamination or metal beads made of abrasion-resistant stainless steel have been used.
In general, the bead that is used by the wet medium pulverization method for the purpose of pulverization or dispersion is made of the material having a higher degree of hardness than the hardness of the raw material to be pulverized.
The beads are driven by rapidly spinning desks of a wet medium pulverizer, for example, a bead mill, so that the beads gain commensurate momentum to move in the slurry at a proper speed. As a result, the beads strike against an inner wall of the vessel or a rotating shaft of the disks, and thereby abrade the inner wall of the vessel or the rotating shaft of the disks. Therefore, the materials of the vessel and the rotating shaft might mix in the slurry and contaminate the raw material to be pulverized. In addition, the beads collide with each other and are subject to wear. Therefore, the materials of the beads might also mix in the slurry.
Japanese Patent Publication No. 03-068444 (JP03-068444A) teaches that a process of charging fine powder having the particle size of below 100 μm, for example, below 10 μm into a bath of cryogenic or cryoscopic liquid prevents the particles of the fine powder from cohering and can mix a different kind of powder particles homogeneously.
Japanese Patent Publication No. 2001-046899 (JP2001-046899A) discloses a continuous circulation type bead mill, which is adapted to prevent the abrasion of a vessel etc. of a wet type media grinding machine, comprising a plurality of stirring members disposed in a cylindrical stirring tank and arranged at predetermined intervals apart from each other, a stirring part for agitating bead-like dispersion media filled in the stirring tank and a slurry-like ground material to be injected into the stirring tank, a centrifugal separation part arranged above the stirring part to centrifuge the dispersion media from the ground material and take the ground material out of the stirring tank, and means for preventing the abrasion of an upper surface of the centrifugal separation part and an inner wall of the stirring tank.
Japanese Patent Publication No. 2002-306940 (JP2002-306940A) discloses a continuous circulation type bead mill, which is adapted to use dispersion beads having very small particle size without causing any clogging with undispersed pigment particles and any wear by the dispersion beads, wherein a flow passage is formed to extend from an annular space defined by an inner wall of a vessel and an outer peripheral surface of a rotor to a discharge port of the vessel through the inside of the rotor, a centrifuge is arranged at an intermediate position of the flow passage in the rotor, the bead mill is used for centrifugally separating the dispersion beads from dispersion-treated paste due to the centrifugal force created by the rotor of the centrifuge.
Next, in the slurry retained in the vessel (a pulverization chamber) of the wet type media grinding machine, the fine powder created by the wet type media grinding machine is mixed with the beads for pulverization or dispersion. Therefore, when pulverizing other materials by the same wet type grinding machine, it is necessary to take the slurry and the beads out of the vessel to clean the vessel and it would be necessary to make a cleaning operation of the wet type grinding machine and wash out the beads taken out of the vessel.
Japanese Patent Publication No. 2007-268403 (JP2007-268403A) discloses a bead mill adapted to facilitate the maintenance of the grinding machine by minimizing the quantity of the residue slurry in the grinding machine and taking the residue slurry and the small beads out of the grinding machine, easily, completely and in a short time.
As mentioned above, the fine powder of the ground material produced by the wet type grinding machine is mixed with the beads for pulverization or dispersion in the slurry retained in the vessel of the wet type grinding machine. Usually, the beads are separated from the slurry first and then the fine powder is separated. Since the fine powder separated from the slurry is slurry-like substance, it should be subject to a drying process for producing dry powder. If the powder heated in the drying process is reaggregated, the powder should be pulverized or dispersed again.
Japanese Patent Publication No. 2003-1129 (JP2003-001129A) discloses a method for producing fine powder comprising the steps of charging usual beads for pulverization and cryogenic liquefied inert gas in a wet type grinding machine, producing a suspension formed by dispersing the materials to be pulverized in the liquefied inert gas, and pulverizing the materials by agitating the suspension together with the beads and then evaporating the liquefied inert gas to obtain dry powder. Thereby, a conventional dry process can be eliminated when producing dried-fine powder of the material to be pulverized by the wet type grinding machine.